Induction furnace



Sept. 12, 1933. T. H. LONG INDUCTION FURNACE Filed April 2, 1931 2 Sheets-Sheet l INVENTOR 7720777 05 H L 027g.

VWTNESSES:

ATTORNEY Sept. 12, 1933. T, G 1,926,811

INDUCTION FURNACE Filed April 2, 1931 2 Sheets-Sheet 2 57 3 rmmrm fig. 8.

WITNESSES: INVENTOR Thomas HLong.

7 ATTORNEY Patented Sept. 12, 1933 UNITED STATES PATENT OFFICE INDUCTION FURNACE Application April 2, 1931. Serial No. 527,219

11 Claims.

My invention relates to induction heaters and has particular relation to the supply of power for the operation of induction heaters.

It is a well known fact that a polyphase generator may be made considerably smaller than a single-phase generator capable of supplying the same amount of power. On the other hand, the matter of providing a polyphase winding for an inductive heater is not at all simple and tends to complicate the design of the heaterto an considerable extent. Hence, the advantage of a polyphase generator cannot be realized in inductive heating unless some method is provided whereby an inductive heater having a singlephase coil may be supplied with power from a polyphase source in an efilcient manner.

It is, accordingly, an object of my invention to provide an electrical circuit for the operation of an induction heater from a polyphase power source.

Another object of my invention is to provide, for the operation of an induction heater of the type equipped with a coil to be supplied from single-phase power, a circuit whereby the heater shall be operated from a polyphase-power source.

A further object of my invention is to provide for the operation of an inductive furnace of the type incorporating a coil to be supplied from a single-phase power line, a circuit whereby the coil shall be so supplied from a polyphase-power source that the power supplied at the terminals of the source shall be utilized to the best advantage.

Still another object of my invention is to provide a polyphase-power system of the type adapted to supply power to a single-phase utilization circuit.

A more specific object of my invention is to provide, for a polyphase power system of the type adapted to supply power to a single-phase utilization circuit, a coupling device whereby the other phases of the power source shall feed power into the phase that is directly coupled to the utilization circuit.

. Another specific object of my invention is to provide, for a polyphase-power system of the type adapted to supply power to a single-phase utilization circuit, a unitary device for coupling the other phases of the pow-er source to the phase directly connected to the utilization circuitand for increasing the power factor of the energyimpressed on the utilization circuit.

A further specific object of my invention is to provide a power circuit for operating an inductive heater from a polyphase source with sub- (Cl. 2l947) stantially the same equipment as is necessary for its operation from a single-phase source.

More concisely stated, it is an object of my invention to provide an electrical system for emciently and inexpensively operating an inductive heater of the type including a single-phase heating coil from a polyphase-power source.

According to my invention, I provide an electrical system for supplying power to a singlephase inductor of the type wherein the terminals of one phase of the power source are connected directly to the inductor, while the remaining terminals of the source are coupled to the directly connected terminals through a plurality of capacitors. The magnitude of the capacitors is so adjusted, relative to the frequency of the power source and to the magnitude of the inductor, that the power factor is materially increased by the coupling.

Specifically, I have provided for the operation of an inductive heater from a three-phase source that is, in turn, transformed into a two-phase power-supply source by a Scott-connected transformer. The coil of the inductive heater is connected across one secondary of the Scott transformer, while the other secondary is coupled to the phase connected to the inductor through a capacitor.

I have found, moreover, that, if the voltage impressed across the inductor is large, as compared with the voltage of the other phase, the power factor may be corrected by the coupling capacitor alone. By properly adjusting the relative magnitude of the voltages of the two phases, the additional capacitor that is ordinarily utilized for increasing the power factor of a singlephase system, and which would be utilized for the ordinary polyphase system, may be eliminated.

It is also well to note that, in systems of this type, the load distribution between the two phases may beregulated by properly adjusting the voltage of the coupled phase or by varying the magnitude of the coupling capacitor. In many instances, the former method is preferable, in view of the fact that it is highlylundesirable to permanently keep a number of expensive capacitors at the disposal of the operator of an inductive heater. Moreover, when the capacitor is changed, readjustment of the power factor is required and,

consequently, readjustment of the whole circuit 105 is necessary. The necessity for a system for regulating the load distribution arises by reason .of the fact that the heater load varies over a considerable range, according to the character of the 1 heated charge.

The novel features that I consider characteristic of my invention are set forth with particu larity in the appended claims. The invention itself, however, both as to its organization and its method of operation, together with additional objects and advantages thereof, will best be understood from the following description of speciflc embodiments, when read in connection with the accompanying drawings, in which:

Figure 1 is a view, in section, showing an inductive heater to which my invention has been applied,

Fig. 2 is a diagrammatic view showing an electrical circuit provided for'the operation of an inductive heater constructed and operated according to the practice of my invention,

Figs. 3 and 4 are vector diagrams showing the relation of the voltages and the currents of the.

circuit illustrated in Fig. 2,

Figs. 5 and 6 are vector diagrams showing the relation of the voltages and the currents of a circuit, such as is illustrated in Fig. 2, but wherein the magnitude of the voltage impressed across the coil of the inductive heater is large as compared with the magnitude of the voltage impressed across the terminals of the other phase,

Fig. 7 is a diagrammatic view showing a modification of my invention, and

Fig. 8 is a vector diagram showing the relation between the voltages and the currents associated with the circuit illustrated in Fig. '7.

The apparatus shown in Fig. 1 comprises a cylindrical refractory container 1 wherein a charge to be heated is deposited. The heating inductor 3 is a solenoid embedded in an insulating medium 5 surrounding the refractory container 1. 'The inductor 3, the container 1 and the insulating medium 5 are disposed in a metallic vessel '7. The refinements commonly incorporated with apparatus of the type illustrated herein have not been shown, since those have no relation to the present invention.

The circuit shown in Fig. 2 comprises a threephase power line 9 that is transformed into a twophase power line 10 by a Scott-connected transformer 11. The terminals 12 and 13 of the respective secondaries 15 and 16 are connected to- I gether to constitute a neutral lead.

The inductor 3 of the inductive heater is connected across one secondary 15 of the Scott transformer 13 while a coupling capacitor 1'7 is connected between one terminal 19 of the other secondary 16 of the Scott transformer and a terminal 23 of the first-named secondary 15. A capacitor 25 is provided for correcting the power fac tor of the energy supplied to the inductor 3 and is connected in parallel with the inductor.

In Fig. 3, the voltage supplied across the heating inductor 3 is represented as a vertical vector 2'7, while the voltage across the terminals 13 and 19 of the other phase is represented as a horizontal vector 31. The voltage impressed across the coupling capacitor 1'7 is then vec-torially represented by a line 33 connecting the terminals of the horizontal and vertical vectors 27 and 31. The current passing through the capacitor 17 is represented by a vector 35 perpendicular to the vector 33 representing the differencebetween the voltages delivered by the two phases.

In Fig. 4, the voltages impressed across the two phases are illustrated'in a similar manner to the corresponding voltages in Fig. 3. The current flowing through the inductor 3 lags phase re1a tive to the voltage impressed across the inductor and is, consequently, illustrated by a vector 37 at a predetermined angle to the vector 2'7. The current through the capacitor 25, provided for correcting the power factor, leads the voltage impressed across it in phase by 90 and it is, consequently, represented by a vector 39 parallel to the vector 31, representing the remaining phase, but in an opposite direction thereto. The coupling current is represented, in this view, by the vector 35 drawn from the end of the vector 39 representing the current through the capacitor 25 provided for correcting the power factor. The current flowing in the secondary 15, whereby the inductor 3 is fed, is represented by the sum of the three current vectors 3'7, 39 and 35; that is to say, by the vector 41.

It is to be noted that the vector 41 is considerably smaller than it would be if the current were supplied from the secondary 15 only, the coupling capacitor being omitted. In the latter case, the current vector would be equivalent to til; sum of the vectors 3'7 and 39.

'1gs. 5 and 6 correspond, respectivel to Fi s. 3 and 4. The voltage impressed acro s the i ductor coil 3, in this case, however, is large, as compared with the voltage impressed across the terminals 13 and 14 of the other phase. The reiagtotn betwteen the voltages can be seen from wo vec ors 4 resented. 3 and 45 by which they are rep- The current through the ca acitor 1 D d y a vector 47 at right anglzs io ta diiference between the vectors 43 and 45 representmg the voltages, and it will be noted that the angle to the horizontal of the vector 47 is considerably smaller than the angle of the corresponding vector 35 in Figs. 3 and 4. I

From Fig. 6, it can be seen that the vector 47, when added to the vector 49, representing the current flowing through the inductor 3, produces substantially the same result as the two vectors 35 and 39 illustrated in Fig. 4. The resultant vector 51, representing the current flowing in the secondary 15 of the Scott transformer 11, is equal, in magnitude and direction, to the corresponding vector 41. Hence, if the magnitudes of the two voltages are properly adjusted, relative to each other, and if a capacitor 1'7 of suflicient magnitude is provided for coupling the two phases, the power-factor-correcting capacitor 25 is not necessary.

It is to be'noted that the load distribution between the two phases may be regulated by properly adjusting the voltage represented by the vector 45. This object is accomplished in a simple manner by providing the coils of the Scott transformer with appropriately distributed taps.

In Fig. 7,. apparatus of the type incorporating a four-wire, two-phase system 53 is illustrated. In this system, the inductor coil 3 and the powerfactor-correcting capacitor 25 are connected across the two terminals 55 and 5'7 of the secondary 15 of the Scott transformer 11. The remaining terminals 59 and 61 of the system are symmetrically coupled to the termials 55 and 57, respectively, by capacitors 63 and 65 of appropriate magnitudes. It is to be noted that, if it is found to be desirable, the mid-taps of the secondaries 15 and 16 of the Scott transformer 11 may be joined together and connected to ground.

In Fig. 8, the relation between the voltages and the currents of the system illustrated in Fig.

'7 is represented vectorially. The voltage vectors 6'7 and 69 are shown as intersecting. The point of intersection of the vectors 67 and 69 represents the point of common potential of the two secondaries 15 and 16. If the mid-taps are connected, the point of intersection represents the potential of the mid-taps. In such case, the terminals of the secondaries 15 and 16 are traversed by voltages displaced 90 in phase from each other at any instant.

As in the other vector diagrams, the current through the coil 3 and the capacitor 25 are represented by proper vectors 71 and '73. The current flowing in the coupling capacitors 63 and 65 is shown as a common vector '75 and it is seen that it has substantially the same effect as the current flowing in the auxiliary phase of apparatus of the type illustrated in Fig. 2, the vector '77, representing the current flowing in the feedingleads 55 and 57, being of substantially the same length as the vectors 41 and 51.

It is to be kept in mind that numerous modifications of my invention are possible. For example; I have found that the inductor 3 and the capacitor 1'7 of apparatus of the type illus trated in. Fig. 2 may be interchanged. However, if this interchange is made, the voltage represented by the vector 27 or the vector 43, in Figs. 3, 4, 5 and 6. must be opposite in direction to that illustrated. It is apparent that the direction of the vectors is determined by the polarity connections of the transformer, and these may be regulated by the operator.

It is apparent that my invention also has application in a system of the type incorporating a polyphase power source and a coil to be fed from a plurality of phases less in number than the number of phases of the source. It will also be noted that the same arrangement of capacitors may be employed where the singlephase system is the source of power and the polyphase system is a polyphase load; and that the load may be of any type.

It may also be pointed out that a dynamoelectric machine'having a squirrel-cage winding adapted to run at'or close to synchronous speed, e. g. a lightly-loaded induction motor or a synchronous motor with a squirrel-cage winding on its field-poles, may advantageously beconnected to the polyphase system comprising the transformer secondaries, or even to the polyphase primary circuit, and, if this is done, it is well in the purview of 'my invention to dispense with capacitors 1'? of Fig. 1 and 63, 65 of Fig. '7, when so desired. Where such dynamo-electric machines are connected to the polyphase primary system, the secondaries 16 may be dispensed with also. If the dynamo-electric machines are of the synchronous type, they may be overexcited to correct power factor and the con- 1. Inductive heating apparatus including an' inductor to be energized by single phase power supply, a polyphase power source, means for impressing a single phase of said power source across said inductor and means for coupling the other phases of said source to said impressed nngle phase. I

2. Inductive heating apparatus including an inductor to be energized by single-phase power supply, a polyphase power source, means for impressing a single phase of said power source across said inductor and electrostatic means for coupling the other phases of said source to said impressed single phase.

3. Inductive heating apparatus including an inductor to be energized by single-phase power supply, a polyphase power source. means for impressing a single phase of said power source across said inductor and a capacitor for coupling the other phases of said source to said impressed single phase.

. l. Inductive heating apparatus including an inductor, a polyphase power source, means for electrically coupling predetermined terminals of id source to saidindu'ctor and unitary means for coupling the other terminals of said source to said predetermined terminals thereof and for increasing the power factor of the energy impressed on said inductor.

Inductive heating apparatus comprising a polyphase power system, an inductor, means for connecting a predetermined number of said phases to said inductor, and electrostatic means for coupling at least one other phase of said system to said first-named phases.

6. Inductive heating apparatus comprising a polyphase power system, an inductor, means for connecting a predetermined number of said phases to said inductor and means for providing suitable load distribution between said operative phases.

7. Inductive heating apparatus comprising a polyphase power system, an inductor, means for connecting a predetermined number of said phases to said inductor and electrostatic means for providing suitable load distribution between said operative phases.

8. Inductive heating apparatus comprising a polyphase power source, a plurality of leads associated with said source, an inductor, means for connecting said inductor to certain of said leads and electrostatic means associated with said leads for regulating the load distribution and the power factor.

9. Inductive heating apparatus comprising a polyphase power source, a plurality of leads associated with said source, an inductor, means for connecting said inductor to certain of said leads and purely electrostatic means, associated with said leads, for regulating the load distributionand the power factor.

10. Inductive heating apparatus including an inductor to be energized-by single-phase power supply, a polyphase power source, one of the phases of said source having a potential output that is substantially greater than the potential output of the other phases of said source, means for impressing said phase across said inductor and means for coupling the other phases of said source to said impressed phase.

11. Inductive heating apparatus comprising a polyphase power system, at least one of the phases of said system having a potential output that is substantially greater than the potential output of the other phases of said system, an inductor, means for connecting said phase to said inductor and electrostatic means for coupling at least one other of the phases of said system to said connected phase.

THOMAS H. LONG. 

